7. Project Summary/Abstract The applicant proposes a two aim strategy to explore molecular mechanisms of the regulation of ion-motive ATPases. Aim 1 focuses on the mechanism of SERCA regulation by phospholamban. This aim will determine how calcium relieves inhibition of SERCA by phospholamban and test the novel hypothesis that there is an undiscovered second binding site on SERCA for phospholamban. The planned experiments will also reveal how phosphorylation alters the localization of the phospholamban cytoplasmic domain on the SERCA cytoplasmic headpiece. There is still no high resolution structural information relating this regulatory mechanism; it is most important unanswered question in the field. Aim 2 will focus on the structural changes of the SERCA pump during the transport cycle. Protein structural transitions will be quantified with advanced fluorescence methods and molecular dynamics simulations. The experiments will provide information about how the physical motions of pumps are important for their biological function.